The construction of Sun clocks is based on an extremely ancient science known as gnomonics. Largely used in ancient Greece and Rome, the sundial consists of a gnomon, or style, which casts the Sun's shadow onto a flat surface marked with lines to indicate the hours of the day. The length and direction of the Sun's shadow depend on the latitude of the dial's location, on the day of the year and the hour at which the observation is made. Vitruvius (1st century B.C.-1st century A.D.), who considered familiarity with gnomonics to be a prerequisite for every architect, describes the rules for constructing the circle of the months and for laying out the hour lines in his De Architectura, when he established the basic elements for the geometrical design of Sun clocks.